Table of contents

The energy dependence of surface reactions has been investigated through ab initio MD simulations for collisions between Al¹⁺ and a gibbsite surface. No change in surface composition was observed for 0 eV initial kinetic energy of Al¹⁺. An increase in energy to 3.5 eV resulted in extended surface migration of hydrogen, subsequent H₂ formation and desorption from the surface. These results may be understood based on thermodynamics and an increase in entropy upon H₂ formation. They are of fundamental importance for an increased understanding of thin film growth through the correlation between ion energy and film composition. They may also indicate a pathway to affect impurity incorporation during film growth.

Colloidal suspension can be regarded as an ideal model system of emulsions, protein solutions, foods, and inks. When there are strong attractive interactions between colloidal particles, they aggregate, phase separate, and sometimes form gel. The basic understanding of the resulting formation of superstructures is of crucial importance from both the scientific and industrial viewpoints. Here we provide clear experimental evidence suggesting that phase separation of colloidal suspensions can take the following kinetic pathway accompanying a metastable transient gel state: upon the phase separation, a percolated network is formed by a hierarchical clustering mechanism even at an extremely low colloid volume fraction (<10⁻³). Then the network structure coarsens with time under the influence of the connectivity and the resulting self-generated mechanical stress. The similarity of this behaviour to droplet-forming viscoelastic phase separation in a dilute polymer solution suggests that colloid phase separation may be classified as viscoelastic phase separation.

In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, T_m, and the glass transition temperature, T_g, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, T_m, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of T_g are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.

We discuss recent developments and present new findings on the structural and phase properties of dipolar model fluids influenced by various external perturbations. We concentrate on systems of spherical particles with permanent (point) dipole moments. Starting from what is known about the three-dimensional systems, particular emphasis is given to dipolar fluids in different confining situations involving both simple and complex (disordered) pore geometries. Further topics concern the effect of quenched positional disorder, the influence of external (electric or magnetic) fields, and the fluid–fluid phase behaviour of various dipolar mixtures. It is demonstrated that due to the translational–orientational coupling and due to the long range of dipolar interactions even simple perturbations such as hard walls can have a profound impact on the systems.

Suspensions of three different clays in K15, a thermotropic liquid crystal, have been studied by optical microscopy and small angle x-ray scattering. The three clays were claytone AF, a surface treated natural montmorillonite, laponite RD, a synthetic hectorite, and mined sepiolite. The claytone and laponite were sterically stabilized whereas sepiolite formed a relatively stable suspension in K15 without any surface treatment. Micrographs of the different suspensions revealed that all three suspensions contained large scale structures. The nature of these aggregates was investigated using small angle x-ray scattering. For the clays with sheet-like particles, claytone and laponite, the flocs contain a mixture of stacked and single platelets. The basal spacing in the stacks was independent of particle concentration in the suspension and the phase of the solvent. The number of platelets in the stack and their percentage in the suspension varied with concentration and the aspect ratio of the platelets. The lath shaped sepiolite did not show any tendency to organize into ordered structures. Here the aggregates are networks of randomly oriented single rods.

It was recently demonstrated that Stillinger–Weber silicon undergoes a liquid–liquid first-order phase transition deep into the supercooled region (Sastry and Angell 2003 Nat. Mater. 2 739). Here we study the effects of perturbations on this phase transition. We show that the order of the liquid–liquid transition changes with negative pressure. We also find that the liquid–liquid transition disappears when the three-body term of the potential is strengthened by as little as 5%. This implies that the details of the potential could affect strongly the nature and even the existence of the liquid–liquid phase.

Heat capacities of (AgI)_x(Ag₃PO₄)_1−x glasses with x = 0.70 and 0.80 were measured to estimate the jump, ΔC_p, in the β-glass transition due to freezing of the Ag⁺ ion rearrangement. Dielectric relaxation properties of (AgI)_x(Ag₂PO_3.5)_1−x with x = 0, 0.20, and 0.40 and (AgI)_x(Ag₃PO₄)_1−x glasses with x = 0.30 and 0.40 were examined to estimate the activation energies, Δε_a, along the conduction path. ΔC_p was found to appear only at larger x than a critical value x_c = 0.60 for (AgI)_x(Ag₃PO₄)_1−x glasses. x_c, depending on the structure of the glass network former, is larger than the values of 0.35 and 0.54 for (AgI)_x(AgPO₃)_1−x and (AgI)_x(Ag₂PO_3.5)_1−x, respectively, glasses. Δε_a was found to show its inflection at around the respective x_c values for the (AgI)_x(Ag₃PO₄)_1−x and (AgI)_x(Ag₂PO_3.5)_1−x glasses. These observations are discussed to support the 'amorphous AgI aggregate' model for the fast Ag⁺ ion conduction path.

Sedimentation–diffusion (SD) equilibria from analytical ultracentrifugation of well-characterized charged silica spheres in ethanol deviate strongly from a barometric profile and demonstrate the existence and substantial effects of a recently predicted internal macroscopic electric field (van Roij 2003 J. Phys.: Condens. Matter15 S3569). Experimental SD-profiles yield the gradient of the electrostatic potential energy of the colloids, which clearly manifests an almost homogeneous macroscopic electric field. Electrochemical Donnan potential measurements confirm a difference in electrical potential between the top and bottom of the profiles. A 'non-barometric' limiting law derived from electroneutrality explains the trends in the SD-profiles quite well. Our analysis of osmotic pressures (obtained from integrating SD-profiles) beyond this simple law includes, among other things, colloid–ion attractions and extra volume terms in the free energy.

We have measured equilibrium sedimentation profiles in a colloidal model system with confocal microscopy. By tuning the interactions, we have determined the gravitational length in the limit of hard-sphere-like interactions, and using the same particles tested a recent theory (van Roij 2003 J. Phys.: Condens. Matter15 S3569), which predicts a significantly extended sedimentation profile in the case of charged colloids with long-ranged repulsions, due to a spontaneously formed macroscopic electric field. For the hard-sphere-like system we find the gravitational length matches that expected. By tuning the buoyancy of the colloidal particles we have shown that a mean field hydrostatic equilibrium description even appears to hold in the case where the colloid volume fraction changes significantly on the length scale of the particle size. The extended sedimentation profiles of the colloids with long-ranged repulsions are well described by theory. Surprisingly, the theory even seems to hold at concentrations where interactions between the colloids, which are not modelled explicitly, play a considerable role.

We address the problem of how liquid is partitioned among Plateau borders in wet two-dimensional foam clusters, and how vertex decoration by Plateau borders changes the bubble areas and the cluster surface energy. We show that the surface energy of wet free clusters of given liquid fraction is lower when the Plateau border pressure is uniform. Furthermore, the surface energy is minimized if the liquid fraction is such that the (uniform) Plateau border pressure equals the outside pressure. Straight decoration of the vertices of a dry foam decreases the area of gas in the bubbles and the total surface energy. If, however, the gas area is kept fixed (by expanding the foam), then the energy may go through a minimum. Detailed results are presented for the case of wet petal clusters.

Instead of spin electrons or holes, the carriers in polymers are supposed to be spin polarons or spinless bipolarons. Based on the spin diffusion theory and Ohm's law, we studied spin-polarized injection into organic polymers. It was obtained that only spin polarons are responsible for the current spin polarization in a ferromagnetic (FM)/organic system. Effects of the conductivity matching and the interfacial resistances on the current spin polarization were discussed.

The local stability of a cylindrical liquid channel or filament deposited on a planar homogeneous substrate is studied in the framework of an effective interface model including the line tension of the three phase contact line. We discuss the stability with respect to transversally symmetric and antisymmetric deformation modes and compute a stability diagram in terms of the contact angle and the longitudinal wavelength of these modes for different values of line tension. An increase in the line tension always leads to an increase in the local stability of liquid channels or filaments. For large positive line tension, the behaviour with pinned contact lines is recovered. As one decreases the line tension to negative values, deformation modes of arbitrary wavelength destabilize the channel or filament for sufficiently small contact angles. In addition, a negative line tension leads to a band of unstable short wavelength modes within the continuum theory considered here. It is argued that the presence or absence of these latter modes depends on the ratio of the contact line width to the molecular size.

Neutron and x-ray diffraction experiments of high resolving power with neutrons from a spallation source and high-energy photons from a synchrotron have been performed on compositional series of binary tellurite glasses with additions of K₂O or P₂O₅ (max. 16 or 32 mol%). Since the P–O bond lengths do not interfere with the Te–O peaks the Te–O and O–O correlations are approximated by Gaussian fitting of the x-ray and neutron correlation functions up to lengths of 0.28 nm. In the case of K₂O–TeO₂ glasses reasonable assumptions are made for the K–O first-neighbour peaks. Te–O and O–O coordination numbers are four and five for the glasses of compositions close to pure TeO₂ which indicates formation of a three-dimensional network of corner-connected TeO₄ trigonal bipyramids. The tellurite network groups are TeO₄ and TeO₃ units in K₂O–TeO₂ glasses and TeO₄ and TeO₅ units in P₂O₅–TeO₂ glasses. Additional Te–O neighbours found at 0.23 nm<r<0.26 nm for K₂O–TeO₂ glasses suggest the existence of TeO₃₊₁ units and those found at 0.23 nm<r<0.29 nm for the P₂O₅–TeO₂ glasses indicate the existence of distorted TeO₆ polyhedra. Tellurite networks are flexible to form the appropriate coordination environments for quite different ions or groups of opposite charge such as K⁺ ions and (PO_4/2)⁻ tetrahedra where change of the TeO_n network units provides the needed number of oxygen neighbours.

Two manufacturing protocols of silicon carbide (SiC) nanowires are discussed. The Raman spectra of produced SiC nanowires are compared with spectra of SiC powders of various grain sizes. The temperature and pressure dependence of the Raman spectra for powders is similar to that of bulk crystals, but is different for nanowires. Frequency shifts, band broadenings and the presence of shoulders are discussed in terms of crystal size, character of defects and their population. The concentration of defects in synthesized nanowires depends on the sintering method. Raman intensity enhancement of the LO phonon was observed when the wavelength of the excitation laser was changed from 780 to 514 nm.

The maximum entropy and reverse Monte Carlo methods are applied to the computation of the phonon density of states (DOS) from heat capacity data. The approach is introduced and the formalism is described. Simulated data are used to test the method, and its sensitivity to noise. Heat capacity measurements from diamond are used to demonstrate the use of the method with experimental data. Comparison between maximum entropy and reverse Monte Carlo results shows that the form of the entropy used here is correct, and that results are stable and reliable. Major features of the DOS are picked out, and acoustic and optical phonons can be treated with the same approach. The treatment set out in this paper provides a cost-effective and reliable method for studies of the phonon properties of materials.

The structural and electronic properties of USn₃ have been calculated in the presence and in the absence of spin–orbit interaction using density functional theory by the Wien2k package. Both the energy band calculation and the density of states curves indicate that spin–orbit interaction has a considerable effect and cannot be ignored. Thus the calculation of the electric field gradient (EFG) as a function of pressure has been performed in the presence of spin–orbit coupling. The contributions of different orbitals to the EFG show that the strongest anisotropy in the charge distribution is due to the electrons in p orbitals.

Exact diagonalization calculations are presented for the t–J model in the presence of a uniform magnetic field. Results for 2 × L ladders (L = 8,10,12) and 4 × 4 square clusters with one and two holes indicate that the diamagnetic response to a perpendicular magnetic field tends to induce a spin liquid state in the spin background. The zero-field spin liquid state of a two-leg ladder is reinforced by the magnetic field: a considerable increase of rung antiferromagnetic correlations is observed for J/t up to 0.6, for one and two holes. Pair breaking is also clearly observed in the ladders and seems to be associated in part with changes promoted by the field in the spin correlations around the zero-field pair. In the 4 × 4 cluster, the numerical results seem to indicate that the field-induced spin liquid state competes with the zero-field antiferromagnetic short range order, the spin liquid state being favoured by higher doping and smaller values of J/t. It is interesting to note that the field effect can also be observed in a 2 × 2 plaquette with one and two holes. This opens up the possibility of gaining a qualitative understanding of the effect.

Emission of electrons from localized electron states in InAs/GaAs self-organized quantum dots (QDs) grown by MOCVD has been studied by a combination of steady-state voltage–capacitance and admittance techniques. We have found a fine structure of carrier concentration profile in the area of QDs, which may be attributed to ground and excited energy states of electrons in the QDs. The total range of activation energies detected in the admittance investigations extends from 20 meV up to 140 meV and testifies to the significant inhomogeneous broadening of the density of state function due to the QD scattering in geometric sizes.

We report x-ray absorption and magnetic circular dichroism measurements performed at the M_4,5 edges of uranium in the ferromagnetic superconductor UGe₂. The spectra are described with the LSDA +U electronic structure computation method. Combined with the analysis of the published (i) x-ray photoemission spectrum, (ii) electron–positron momentum density, and (iii) angular dependence of the de Haas–van Alphen frequencies, we infer for the Coulomb repulsion energy within the 5f electron shell  eV. This leads to where W is the bandwidth.

We report a study of the pressure effect (PE) on the in-plane magnetic field penetration depth λ_ab in YBa₂Cu₄O₈ by means of Meissner fraction measurements. A pronounced PE on λ_ab⁻²(0) was observed with a maximum relative shift of Δλ_ab⁻²/λ_ab⁻² = 44(3)% at a pressure of 10.2 kbar. It arises from the pressure dependence of the effective in-plane charge carrier mass and pressure-induced charge carrier transfer from the CuO chains to the superconducting CuO₂ planes. The present results imply that the charge carriers in YBa₂Cu₄O₈ are coupled to the lattice.

Very detailed measurements of the electrical resistivity of Gd₅(Si_0.1Ge_0.9)₄ are here reported, with special emphasis on the vicinity of the first-order (magnetostructural) martensitic transition which occurs at  K. The data cover more than fifty thermal cycles spanning the temperature ranges of 300–10 K (long cycles) and 105–10 K (short cycles). In the initial 10–300 K cycles the martensitic transition takes place in three closely-spaced steps, with associated resistance (R) discontinuities and large thermal hysteresis. In a subsequent series of short cycles (10–105 K) a unique transition occurs, exhibiting a common and quite reproducible R(T) behaviour within a small temperature range ( K) below T_S, either in heating or cooling runs. Remarkably, this 'local reproducibility' (within ΔT) remains in spite of the significant resistance changes which occur outside the ΔT-range under thermal cycling. In particular the residual resistance systematically increases under thermal cycling, but the corresponding effect is absent in the ΔT temperature range. This excludes microcracking as a dominant resistive mechanism in our results, pointing to an intrinsic character of the reproducible behaviour just below T_S. We also analyse the R(T) behaviour when changing from long to short thermal cycles, and the R(T) evolution towards a reversible final behaviour, after extended thermal cycling.

Magnetic properties of Cr layers in epitaxial Cr(011)/Sn and Fe(011)/Cr/Sn/Cr multilayers with monatomic Sn layers at the centre of the Cr layers were studied by means of ¹¹⁹Sn Mössbauer spectroscopy. It was found that Cr/Sn interfaces tend to enhance the magnetic moments of Cr layers and Fe/Cr interfaces tend to reduce the magnetic moments of Cr layers. These results were compared with those for (001)-oriented multilayers with the same composition. The large reduction of Cr magnetic moments has been found in Fe(011)/Cr/Sn/Cr multilayers and this is attributed to a magnetic frustration effect caused by the intrinsic magnetic structure of ferromagnetic Fe(011) and antiferromagnetic Cr(011) planes at the Fe/Cr interfaces.

We report results of the specific heat of the heavy-fermion compounds of the alloying series CeNi_1−xCo_xGe₂. With increasing x, hybridization between the localized 4f and conduction band electrons is enhanced. The magnetic order observed for the x = 0 composition is completely suppressed at a critical concentration of x_c = 0.3, yielding Fermi-liquid behaviour for x>0.3. We observe significant deviations from the Fermi-liquid behaviour at x_c = 0.3. Anomalies found in the specific heat are well explained by the Kondo model for a degenerate impurity spin J = 1/2,3/2, and 5/2 in the Coqblin–Schrieffer limit for the Co concentration ranges of , , and , respectively.

The structure and the dielectric properties of Pb (Zn_1/3Nb_2/3)O₃ (PZN) crystal have been investigated by means of high-resolution synchrotron x-ray diffraction (with an x-ray energy of 32 keV) and dielectric spectroscopy (in the frequency range 100 Hz–1 MHz). At high temperatures, the PZN crystal exhibits a cubic symmetry and polar nanoregions inherent to relaxor ferroelectrics are present, as evidenced by the single (222) Bragg peak and by the noticeable tails at the base of the peak. At low temperatures, in addition to the well-known rhombohedral phase, another low-symmetry, probably monoclinic, phase is found. The two phases coexist in the form of mesoscopic domains. The ferroelectric phase transition is diffuse and observed between 325 and 390 K, where the concentration of the low-temperature phases gradually increases and the cubic phase disappears upon cooling. However, no dielectric anomalies can be detected in the temperature range of the diffuse phase transition. The temperature dependence of the dielectric constant shows a maximum at higher temperature (T_m = 417–429 K, depending on frequency) with the typical relaxor dispersion at T<T_m and the frequency dependence of T_m fitted to the Vogel–Fulcher relation. Application of an electric field upon cooling from the cubic phase or poling the crystal in the ferroelectric phase gives rise to a sharp anomaly of the dielectric constant at  K and greatly diminishes the dispersion at lower temperatures, but the dielectric relaxation process around T_m remains qualitatively unchanged. The results are discussed in the framework of the present models of relaxors and in comparison with the prototypical relaxor ferroelectric Pb (Mg_1/3Nb_2/3)O₃.

The crystal structures of [C(NH₂)₃]₃[Sb₂Br₉] (Gu₃Sb₂Br₉) at 300 K and of [C(NH₂)₃]₃[Sb₂Cl₉] (Gu₃Sb₂Cl₉) at 90 and 300 K are determined. The compounds crystallize in the monoclinic space group: C 2/c. The structure is composed of Sb₂X₉³⁻ (X = Cl, Br) ions, which form two-dimensional layers through the crystal, and guanidinium cations. In Gu₃Sb₂Br₉ the structural phase transformation of the first-order type is detected at 435/450 K (on cooling/heating) by the DSC and dilatometric techniques. The dielectric relaxation process in the frequency range between 75 kHz and 5 MHz over the low temperature phase indicates reorientations of weakly distorted guanidinium cations. The proton ¹H NMR second-moment and spin–lattice relaxation time, T₁, temperature runs for the polycrystalline Gu₃Sb₂Br₉ sample indicate a complex cation motion. A significant dynamical non-equivalence of two guanidinium cations was found. The possible mechanism of the phase transition in Gu₃Sb₂Br₉ is discussed on the basis of the results presented.